The geographical area covered by a mobile communication system is divided into small radio cells in order to improve the utilization of frequencies. While in a cell, a mobile station communicates with the fixed network via the base station of the cell. Mobile stations can move freely from one cell to another within the system. An example of such a system is the digital mobile telephone system GSM. A mobile station is informed of incoming calls by transmitting via the cell base station a paging message to which the mobile station should respond. Since it is not reasonable to transmit the paging message via all the base stations of the entire system, the fixed network should know the location of the mobile station within a relatively small area. The fixed network typically knows the location of a mobile station within a larger area, generally called a location area, consisting of one or several cells. When the mobile station moves to a new location area, it is registered as a visiting subscriber in the subscriber database of the location area, i.e. in a visitor location register, and registration is simultaneously cancelled in the visitor location register of the previous location area. Furthermore, a mobile communication network usually comprises at least one centralized subscriber database, i.e. a home location register, which stores subscriber data permanently. When a new mobile station is registered in a visitor location register, the subscriber data concerning the subscriber of this mobile station is requested from the home location register, and the subscriber location data in the home location register is simultaneously updated, this data being maintained with an accuracy of a visitor location register. A visitor location register is usually integrated with a mobile services switching centre and it controls several location areas.
When the location of a mobile station is only known with an accuracy of a location area, the mobile station must be paged through all radio cells of the location area in order to establish an incoming call.
A basic problem of radio network planning is how to design base station equipment to correspond to the expected amount of telephone traffic within an area. The average amount of traffic in an area correlates to the average subscriber mobility and density in the same area. However, there may be great variation in the temporary subscriber mobility and subscriber distribution of an area depending on the time of the day.
Due to subscriber mobility, momentary overloading situations may arise at specific base stations, whereupon new calls cannot be established via these base stations. However, there may be base stations with a sufficient amount of free capacity in the same location area. In such a case, paging messages transmitted via overloaded base stations cause unnecessary signalling traffic between a base station and a mobile station in a situation where the base station cannot handle call establishment due to a lack of resources.